Solid-state reactions involve long reaction times and high temperatures, and result in thermodynamically stable products. However, kinetic control is required if a variety of synthetic modifications are desired.

In his Editorial in Angewandte Chemie, Claus Feldmann, Karlsruhe Institute of Technology, Germany, likens negotiating the energy landscape in the synthesis of metastable solids to a climber climbing from one ledge to another. He poses the question about how solid-state reactions can be sped up without increasing the reaction temperature. Among the solutions to this problem are the use of nanoparticles, ionic liquids, gas-phase deposition, and composites and hybrids.
Feldmann also asks the essential question of whether we really need a large number of metastable solids, and points out that the functional materials for applications such as batteries, solar cells, or magnets are all in this category.

• Metastable Solids—Terra Incognita Awaiting Discovery,
  Claus Feldmann,
  Angew. Chem. Int. Ed. 2013.
  DOI: 10.1002/anie.201303750